Ultrasound imaging or other use variable input impedance preamplifier

ABSTRACT

A preamplifier has a variable input impedance. The input impedance is tailored with the signal level to maintain a more optimal performance. The input impedance is varied by at least two amplifiers connected in parallel. Each amplifier has a different input impedance. By controlling the bias currents to the amplifiers, the contribution to the input impedance of the parallel amplifiers is controlled. Gradual variation in input impedance may be obtained by gradual variation in relative contribution by the different amplifiers.

BACKGROUND

The present embodiments relate to variable input impedancepreamplifiers. For example, a variable input impedance preamplifier isused for ultrasound imaging. Other uses may be provided.

For ultrasound imaging, the transducer preamplifier is selected based ona trade-off of input impedance to get as close as possible to desiredperformance. A single impedance amplifier is typically used. Amplifierimpedance and dynamic range are traded with only one optimized for theinput signal. In ultrasound, the signal strength varies over time or asa function of depth from which received echo signals are reflected.

Ultrasound transducers have low impedance at a resonant frequency andhigher impedance away from resonance. There may be less loss of signalsaway from resonance with a high impedance amplifier than with a lowimpedance amplifier due to the dominance of the transducer sourceimpedance in the resistive divider formed between the transducerimpedance and the preamplifier input impedance. This lower loss mayenhance signal-to-noise ratio and bandwidth. With a low impedancepreamplifier, the resistive divider is dominated by the preamplifier,causing higher loss of signals away from resonance and resulting inlower signal-to-noise and reduced bandwidth. The impedance of thetransmit/receive switch may also causes higher losses with lowerimpedance preamplifiers.

Some amplifiers with variable input impedance have been used inultrasound. For example, a switched resistance in a feedback path of afixed gain amplifier is used to control gain, but also varies inputimpedance. Such a switched amplifier topology may change the impedance,but with an interruption in operation for switching.

BRIEF SUMMARY

By way of introduction, the preferred embodiments described belowinclude methods, systems and preamplifiers for providing variable inputimpedance. The input impedance is tailored with the signal level tomaintain a more optimal performance. The input impedance is varied by atleast two amplifiers connected in parallel. Each amplifier has adifferent input impedance. By controlling the bias currents to theamplifiers, the contribution of the parallel amplifiers to the inputimpedance of the preamplifier is controlled. Gradual variation in inputimpedance may be obtained by gradual variation in relative contributionby the different amplifiers.

In a first aspect, a variable input impedance preamplifier is providedfor ultrasound imaging. A lower input impedance amplifier connects inparallel with a higher input impedance amplifier. An ultrasoundtransducer connects with the higher and lower input impedanceamplifiers.

In a second aspect, a variable input impedance preamplifier is provided.A lower input impedance amplifier connects in parallel with a higherinput impedance amplifier. A control circuit connects with the higherand lower impedance amplifiers. The control circuit is operable tocontrol an input impedance of the preamplifier in conjunction with avariance in gain of the preamplifier.

In a third aspect, a method is provided for varying an input impedanceof a preamplifier used for ultrasound imaging. Contribution to the inputimpedance of the preamplifier transitions from a lower input impedanceamplifier to a higher input impedance amplifier as a function of time.The lower and higher input impedance amplifiers connect in parallel.Ultrasound signals are amplified with the preamplifier whiletransitioning.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments and may be later claimedindependently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a circuit diagram of a preamplifier in one embodiment;

FIG. 2 is a circuit diagram of an input stage of the preamplifier ofFIG. 1;

FIG. 3 is a control circuit diagram of one embodiment of a bias voltagegenerator, replicated as part of FIG. 6;

FIG. 4 is a control circuit diagram of one embodiment of a referencegain stage for an input stage;

FIG. 5 is a control circuit diagram of a anti-log stage with temperaturecompensation according to one embodiment;

FIG. 6 is a control circuit diagram of one embodiment of a referencegain stage for a second stage; and

FIG. 7 is a control circuit diagram of one embodiment of a gain controlsplitter with temperature compensation.

DETAILED DESCRIPTION OF THE DRAWINGS AND PRESENTLY PREFERRED EMBODIMENTS

Impedance and gain of a preamplifier are programmably controlled byeither a processor and software or a dedicated circuit. The preamplifiervaries a voltage or current as a function of time or depth (e.g., timegain control (TGC)) in conjunction with varying input impedance.

The embodiments below are for a preamplifier with controllable inputimpedance and circuits to control the transition of the impedance. Thepreamplifier has two input amplifiers connected in parallel. Oneamplifier has a relatively low input impedance, and the other amplifierhas a relatively high impedance. The input impedance variably crossesbetween the lower and higher input impedance amplifiers. When connectedwith an ultrasound transducer, the higher input impedance providesbetter SNR and wider overall bandwidth, and the lower input impedanceprovides higher dynamic range and larger signal handling capability. Byselecting the gains and the contribution to input impedance, both timegain control and an impedance shift may be accomplished. At close rangein ultrasound, the gain and input impedance of the input amplifiers arelow to accommodate the large reflected signal levels. As the propagationdepth increases, the gain and impedance are raised to improvesensitivity and SNR.

A significant effect of input impedance is transducer pulse response(i.e. bandwidth). Generally, for ultrasonic probes, a high impedancepreamplifier provides faster pulse response. As a generalization, thelow input impedance preamplifier provides smoother pulse shapes andgenerally lower axial sidelobes.

In the embodiments shown in FIGS. 1-7, different resistor, voltage, andcapacitor values are shown. Other values may be used. Differentamplifiers or transistors may be used. A complimentary topology could beused by substituting PNP transistors for NPN transistors. Otherimpedance crossover control functions and circuits could be used as wellas different scaling factors on the impedance ratios. The secondary gaincontrol differential pairs could be substituted with other gain controlmethods. Other connections or networks may be used. The circuits areformed on a same integrated circuit or separate integrated circuits.Alternatively, one or more of the circuits or components are discrete.

FIG. 1 shows a variable input impedance preamplifier. The preamplifieris used for ultrasound imaging, such as being connected between anultrasound transducer and a receive beamformer. One preamplifier isprovided for each ultrasound transducer and receive beamformer channel.Other uses may be provided. The preamplifier combines two or more typesof input amplifiers, lower and higher input impedance amplifiers. Eachinput amplifier is operational in a gain range where the attributes ofthe amplifiers are most desirable. At low gains, the lower inputimpedance amplifier is used primarily to maximize large input signalhandling. At high gains, the higher input impedance amplifier is usedprimarily to improve the noise performance. By varying the bias currentsto the higher and lower input impedance amplifiers, it is possible tovary smoothly from low gain and low input impedance to high gain andhigh input impedance. In the middle range, both lower and higher inputimpedance amplifiers are active. The gain and input impedance varyingaccording to the amount of bias current flowing in each of the lower andhigher input impedance amplifiers.

FIG. 1 shows the preamplifier with an ultrasound transducer, atransformer T1, an input (first) stage, an output (second) stage, and adifferential amplifier U3A. Additional, different or fewer stages orcomponents may be used. For example, the input stage is provided withoutthe second stage. As another example, a non-differential configurationis provided without the transformer T1 or differential amplifier U3A.

The ultrasound transducer is a piezoelectric element. A piezoelectricblock or a composite material may be used. Electrodes are positioned onopposite sides of the piezoelectric material. For receive operation,acoustic energy causes the piezoelectric material to expand or contract,generating electrical differential across the electrodes. One electrodeis grounded or held at a desired potential. The other electrode connectswith the input stage. In particular, the electrode connects directlywith the transformer T1 and indirectly to the transistors Q1A/B, Q2A/B,and Q3A/B of the input stage.

FIG. 2 shows the input stage and the transformer T1 of the preamplifierof FIG. 1. The transformer T1 has an input connected with the ultrasoundtransducer. Two outputs each connect with the lower and higher inputimpedance amplifiers formed from the transistors Q1A/B, Q2A/B, andQ3A/B. The turns ratio is 1:2 to provide lower noise for higher gains inthe transducer source impedance range. The transformer T1 increases thegain control range of the input stage by providing a 1:2 voltage step upand a 2:1 current step down. As input stage operation varies fromvoltage mode (common-emitter) to current mode (common-base), the gain is2 in voltage mode and ½ in current mode, giving a 4× gain change. Otherturns ratios may be used. The transformer T1 converts the single-endedinput signal into a differential signal. In an alternative embodiment,the preamplifier and/or input stage is single ended.

The input stage includes transistors Q1A/B, Q2A/B, and Q3A/B. In theinput stage, a higher input impedance amplifier is provided by thetransistors Q1A/B, Q2A/B. The transistors Q1A/B, Q2A/B form adifferential amplifier in a common emitter configuration. The inputimpedance of the transistors Q1A/B, proficient Q2A/B is about 450ohms∥20 pF, but other input impedances may be provided. Two parallelpairs of transistors Q1A/B, Q2A/B reduce noise, such as by reducing theeffective base resistance of the input pair. Other higher inputimpedance configurations or components may be used.

Also in the input stage, a lower input impedance amplifier is providedby the transistors Q3A/B. The transistors Q3A/B form a differentialamplifier in a common base configuration. The input impedance is about25 ohms or less, but greater values may be provided. Lower and higherare used as relative terms. The gain of the lower input impedanceamplifier is dependent on the source impedance of the ultrasoundtransducer. Thus, the gain control range of the input stage is dependenton the source impedance of the ultrasound transducer.

The lower and higher input impedance amplifiers connect in parallel. Theoutput from the transformer T1 connects with the transistors Q1A/B,Q2A/B and Q3A/B. The outputs from the transistors Q1A/B, Q2A/B and Q3A/Bconnect the lower and higher impedance amplifiers with the second stage.

In general, there are three ranges or regions of operation of thevariable-input-impedance input stage of the preamplifier. Low gain andinput impedance are provided by only or primarily using the lower inputimpedance amplifier. The bias provided to the higher input impedanceamplifier is a sufficiently low value to turn off the correspondingtransistors (Q1A/B and Q2A/B). High gain and impedance are provided byonly or primarily using the higher input impedance amplifier. The biasprovided to the lower input impedance amplifier is a sufficiently lowvalue to turn off the corresponding transistors (Q3A/B). Gains and inputimpedances between the maximum and minimum for the input stage(crossover region of operation) maintain operation or biases for boththe higher and lower input impedance amplifiers. By varying the biascurrents to the lower and higher input impedance amplifiers, the inputstage may gradually transition between the lower and higher inputimpedance amplifiers.

Referring again to FIG. 1, the second stage includes a current steeringpair of transistors Q4A/B, Q5A/B. The collector currents of the inputstage connect with the current steering pair of transistors Q4A/B,Q5A/B. The input stage varies a gain over a first range. The secondstage varies the gain over an additional range to achieve a totaldesired range, such as a total of 50 dB. The output of the second stageis converted to single-ended with the differential amplifier U3A.

In the embodiment shown in FIG. 1, the noise with a source impedance of50 ohms is 1.7 dBNF. At 100 and 150 ohms, the noise is less than 1 dBNFat a spot frequency of 5 MHz. These noise includes any transmit/receiveswitch with a diode bridge in a return leg of the input transformer T1(not shown). Greater (e.g., about 3 dB) or lesser noise may be provided.

The second harmonic distortion is less than −60 dBc with a 5 MHz carrierat full output in pulsed modes, but may be greater. Over useful signallevels, the preamp may achieve >60 dB rejection of second harmonic witha 5 MHz carrier. At extremely large input signal levels, less rejectionmay be provided. For large input signal levels, the second harmonicrejection may not be as important, since there may be no second harmonicsignal of interest at large amplitudes. At shallow depths associatedwith larger signals, the signal has not propagated far enough togenerate any significant second harmonic information.

The range of gain control is approx 50 dB, but greater or lesser rangemay be provided. Two gain controls are provided in the preamplifier. Theinput stage has a gain control range that depends on the sourceimpedance, and the second stage has a practical gain range of >60 dB. Inthis embodiment, the input stage may have 16 dB of gain control rangewith a source impedance of about 110 ohms. For source impedances aboveor below 110 ohms, the gain range may be proportionately larger orsmaller, respectively. By limiting the second stage to a gain range of34 dB, the total gain range of the preamplifier is 50 dB.

Input signal handling in pulsed modes is about 4 mAp-p (short circuitcurrent) at minimum gain, but other values may be provided. With thesecond stage gain control at a minimum gain of −36 dB, and the inputstage at minimum gain, the input signal handling may be >6 mApp. At 6mApp, the output second harmonic component (5 MHz carrier) is about −58dBc and the third harmonic is about −50 dBc. At 2 mApp input, and thesecond stage gain set to give a full output signal level, the secondharmonic level is <−65 dBc and the third harmonic is about −52 dBc

The power consumption less may be less than 50 mW per receive beamformerchannel, including any transmit/receive switch. The power consumptionvaries with the input stage gain setting. At high gain, the powerconsumption may be 58 mW, and at low gain, the power consumption may be60 mW. Greater or lesser power consumption may be provided. Power istaken from a 5V power supply, but other voltage supplies may be used. Incontinuous wave (CW) mode, the input stage bias current is set higherfor higher signal handling, and the total power may be about 100 mW.

For discrete circuit embodiments, such as that disclosed here, the sizeof the preamplifier may be less than about 0.75 in². A preamplifierboard with four channels may be laid out on a PC board of about 3 in²,including connectors, supply bypassing, transmit/receive switch, CWlogic and switching circuitry not shown in FIG. 1. Greater or lessersizes may be used.

The ultrasound receive beamformer channel includes any further time gaincontrol amplifiers, delays, phase rotators, mixers, summers, modulators,base band filters, combinations thereof or other components forbeamforming from ultrasound signals received from a plurality of thepreamplifiers and associated ultrasound transducers. The receivebeamformer channel connects with the lower and higher input impedanceamplifiers, such as an indirect connection through the second stage.

A control circuit or circuits control the gain and input impedance ofthe preamplifier. Bias currents of the higher and lower input impedanceamplifiers (transistors Q1A/B, Q2A/B, and Q3A/B) are varied. Asultrasound signals from greater depths (increasing amount of time) arereceived, the amplitude of the signals decreases. The gain is increasedas a function of time to compensate for the greater attenuation of thesignals. The input impedance is also increased for desired preamplification attributes, such as signal to noise ratio and bandwidth.Gain and impedance may be decreased as a function of time.

The control circuit is a processor, integrated circuit, discrete circuitor other device. In the embodiment of FIG. 1, the preamplifier includesthree control inputs, vg1, vg2 and vb. One control input vg1 inconjunction with the control input vb controls the gain and associatedimpedance of the input stage. Another control input vg2 controls thegain of the second stage. The control input vg1 controls the bias of thehigh input impedance amplifier. The combination of vb and vg1 controlsthe bias of the low input impedance amplifier. Additionally, differentor fewer control inputs may be provided.

The control circuits are responsive to a single time gain control input,TGC. The TGC signal is split to provide separate time-gain controlsignals, TGC1 and TGC2, for the input and output stages (see FIG. 7).TGC1 passes through an anti-log stage with temperature compensation forcontrolling a gain current Igain (see FIG. 5). Igain connects with areference gain stage for controlling the gain of the input stage withthe vg1 and vb signals (see FIGS. 3 and 4). The vg1 signal is alsoresponsive to the vb signal (see FIG. 4) as well as used to generate thevb signal (see FIG. 3). For the second stage, TGC2 connects with areference gain stage outputting vg2 (see FIG. 6). vg1 and vb are alsoprovided to the output stage reference gain control circuit to bias thecircuit in a way that is identical or similar to the circuit it iscontrolling (see FIG. 6). Other combinations of circuits with the sameor different interrelationship of control signals may be provided.Additional, fewer or different control signals and/or control circuitsmay be provided, such as using the input stage and associated controlcircuits without the second stage and associated control circuits.

The control inputs are independent of each other or may be dependent onone or more other control signals. In one embodiment, vb is a constantbias voltage and TGC1 is varied to control gain and input impedance. Inanother embodiment, vb depends on vg1. The base of the lower inputimpedance amplifier is driven by vb as an attenuated and voltage levelshifted signal as compared to vg1. Gain control and input impedancecontrol of the input stage are both dependent on how signals vb and vg1are driven. The dependence of vb on vg1 disclosed here increases thevoltage change on vg1 required in order to turn the common-base lowerinput impedance amplifier on and off. The level shifting is partly avoltage shift and partly a reference junction base-emitter voltage drop.The motivation for doing this is to match the voltage change of vg1required to turn off the low input impedance amplifier to the changerequired to turn on the high input impedance amplifier. Differentsources of level shifting may be provided.

FIG. 3 shows one embodiment of a control circuit for generating thevoltage or current bias, vb. Amplifiers U4A/B buffer nodes in thecircuit that otherwise would interact in an undesirable way. Thiscontrol circuit performs level shifting and attenuation of vg1.Generally, Q6A and Q6B may mimic the biasing arrangement of the lowerand higher input impedance amplifiers in the crossover region ofoperation. The amount of current flowing in Q6A/B may be reflected inthe amount of current flowing in the moderately biased lower and higherinput impedance amplifiers. The attenuation of the input vg1 reflectedin vb expands the voltage range over which vg1 must swing to transitionthe lower input impedance amplifier from off to on, in order to matchits range to that required to transition the higher impedance amplifierfrom on to off. The voltage shift and attenuation may be determinedempirically, considering the current flowing in each of the lower andhigher input impedance amplifiers separately and then fitting themtogether to provide a smooth crossover and reasonably constant totalbias current from the common-emitter region, through the crossover tothe common-base regions. The bias current of the higher input impedanceamplifier may be somewhat higher at high gains to improve noise figure.

The control circuit also acts as a temperature compensation circuit. Thecontrol circuit of FIG. 3 is included in or on the preamplifier module.The close proximity to the input stage may provide better temperaturecompensation. In a similar way, the equivalent circuit is included inFIG. 6 to act as temperature compensation to the input stage biascurrent mimicking transistors Q5A/B, Q6A/B, and Q7A/B. For this reason,Q8A and Q19B are kept in close proximity to Q5A/B, Q6A/B, and Q7A/B.

The input impedance of the preamplifier is controlled in conjunctionwith a variance in gain of the preamplifier. The gain of thepreamplifier varies as a function of the bias currents. FIG. 4 shows thecontrol circuit for controlling the input stage of the preamplifier. Theinput stage is controlled by placing a reference gain stage in afeedback loop, configured in the same way as the input stage, andoperating at the same bias levels. The reference gain stage mimics theoperation of the input stage of the preamplifier in the feedback loop.The feedback loop acts to extract the control signal required to balancea variable input offset, Igain, with an opposite polarity, fixed outputoffset. The feedback loop senses the net output offset and drives thegain of the input stage to equalize the two offsets, creating a netoutput offset of zero. The gain signal vg1 is used to drive multiplepreamplifiers or channels, which then operate at the same gain as thereference channel. By varying the input offset Igain, the reference gainstage is driven to a variety of gains. The ratio of the output offset tothe input offset is the gain.

Q12A/B, Q13A/B and Q14A/B mimic the input stage of the preamp. The inputoffset Igain is input by the operational amplifiers U15A/B. Theoperational amplifiers U15A/B receive a current based gain controlsignal (Igain) and drive a differential signal across the input nodes ofthe reference stage transistors Q12A/B, Q13A/B and Q14A/B. Thecommon-mode operating point for operational amplifiers U15A/B is sensedfrom the reference stage, and so their outputs float to the operatingbias point of the reference stage. In this way, the amplifiers U15A/Bdrive only the differential voltage of the reference input, not thecommon-mode voltage. The operating point of the reference and inputstages move up and down in common-mode voltage depending on the gainsetting.

The resistors R104 and R105 together mimic the 33.2 ohm transformercenter-tap resistor (R14) in the preamplifier (see FIG. 1). They aresplit into two parallel resistors to accommodate feeding in the inputoffset through resistors R86 and R89. The resistors pairs R94, R95 andR107, R108 sense the common-mode voltage of the two nodes to which theyare tied.

The value of resistors R109/R107 and R110/R108 are chosen to set theeffective source resistance of the input offset signal driven into thereference stage by U15A/B. They are analogous to the source impedance ofthe transducer feeding the input stage. At minimum gain, the gaindepends on the current into the common-base pair emitters (Q12A/B) whichis the open-circuit voltage divided by the source impedance. Since thegain of the low input impedance amplifier depends on the sourceresistance, the lowest gain and therefore the gain control range of theinput stage both depend on the value of the source impedance. At highgains, the input impedance of the input stage is relatively high so thegain is relatively independent of the source impedance, and theamplifier responds primarily to the open-circuit voltage of the source.The source resistance set primarily by R104, R105, R107, R108, R109 andR110 is fairly representative of a typical transducer impedance, afteraccounting for the 4× impedance step-up of the input transformer T1. Theresistance of the resistor network between the input nodes, includingR86, R89, R104, R105, R107, R108, R109, and R110 is about 400 ohms. Thisvalue is similar to a transducer source impedance of about 100 ohmsgiven the 4× impedance step-up of the transformer T1.

At maximum gain in the input stage, the gain is relatively independentof the source impedance of the ultrasound transducer. At minimum gain inthe input stage, the gain is approximately inversely proportional to thesource impedance of the ultrasound transducer. The nominal gain controlrange of the input stage is 16 dB with a source impedance of about 100ohms, but other ranges may be provided. For a transducer sourceimpedance of 50 ohms, the input stage gain control range isapproximately 10 dB because the low gain is approximately 6 dB higherthan if the source impedance were 100 ohms.

The output offset is fed in by the transistor Q18A. U17A detects the netoutput offset and drives the vg1 node to drive the gain of the referencestage to equalize the input and output offsets. vg1 is output throughthe buffer U9B as a preamplifier input stage gain command. vg1 is sensedas the common mode voltage at 33.2 common-mode ohms away from the inputnodes where vg1 is fed into the preamplifier. Other values may be used.

In operation, the loop error amplifier, U17A, drives the loop toeffectively zero the voltage difference between the input nodes of theloop error amplifier U17A. The currents flowing in the collectorresistors, R91 and R93, are about equal, but the currents flowing in thetwo halves of the reference gain stage are unequal by the current thatflows into the collector of transistor Q18A. The transistor Q18Aprovides the output offset. Because of this differential current in thereference input stage, the transistors Q12A/B, Q13A/B and Q14A/B of theinput differential pairs dissipate different amounts of heat. Thetransistors Q12A/B, Q13A/B and Q14A/B are matched, but may not bethermally coupled. Q15A holds the collectors of transistors Q13B, Q14B,and Q12A at a lower collector-to-emitter voltage to offset the effectsof the extra collector current being carried, approximately equalizingthe power dissipation of the transistors Q13B, Q14B, and Q12A totransistors Q13A, Q14A, and Q12B. Transistors Q15A/B, resistor R92 andamplifier U17B hold the collector voltage at vb, substantiallyequalizing the power dissipation over the gain control range.

The bias signal, vb, controls the bases of the common-base pair oftransistors Q12A/B. The circuit used to generate vb is the same as thecircuit used to generate vb for the preamplifier (i.e., FIG. 3), butdifferent circuits may be used.

Because the gain control is linear or controls gain as a ratio, notlogarithmically, the gain signal Igain is linear. The TGC commandvoltage in ultrasound is typically logarithmic (in dB/V), but may belinear. The logarithmic TGC command voltage is converted to a linearsignal in one embodiment. FIG. 5 shows such an anti-log control circuit.The anti-log control circuit uses the exponential relationship ofcollector current to base-emitter voltage for a bipolar junctiontransistor. This relationship is temperature dependent. Temperaturecompensation is provided by a NTC thermistor RT3 in conjunction withR146, R147, R148, R149 and R150 to compensate the base drive voltage,optimized for the range 25 C to 60 C. Alternate methods may be used,such as piecewise linear approximation.

The anti-log control circuit includes a matched pair of transistorsQ304A/B to generate the anti-log function. The collector current intransistor Q304A (Iref) is set by R152. The emitter voltage of Q304A isbuffered by U86B and driven into the emitter of the anti-logging outputtransistor Q304B. The base of the transistor Q304B is driven by thevoltage-divided input. For a TGC1 input of 0V, the collector currentIgain is the same as Iref. The gain factor for TGC1 is 20 dB/V, butother values may be provided. The gain factor is a scale factor thatrelates TGC1 to Igain above or below 0V. More precisely,Igain=Iref(10^(TGC1)). With the circuit shown, the gain of the inputstage decreases for increasing values of the TGC1 command voltage,because the gain of the reference stage is equal to the output offset (afixed value) divided by the input offset, and the input offset is Igain,so as Igain increases, stage gain decreases.

The collector of the output transistor is in cascade with Q36 and R151to approximately normalize the power dissipation in the outputtransistor Q304B to that of the reference transistor Q304A. Thisequalization is approximate. The equalization may be avoided or notprovided where there is good thermal coupling between transistorsQ304A/B.

The control circuits of FIGS. 3-5 control the gain of the input stage.The gain of the input stage is a function of input impedance. Anothercontrol circuit controls the gain of the second stage of thepreamplifier. In general, there are two control circuits, one for eachgain control stage of the preamplifier. The two gains are adjusted at asame time, at different times or combinations thereof. For example, thegain of the input stage is varied first from a minimum to a maximum. Thegain of the second stage is varied after the gain of the input stage isat a maximum. The gain of the second stage is varied from a minimum to amaximum. Alternatively, the gain of the second stage is varied beforethe input stage.

FIG. 6 shows the control circuit for controlling the second stage(current-steering amplifier) of the preamplifier. The maximum currentgain of the second stage is 1. The gain is controlled by thedifferential base voltage applied to the transistors Q4A/B and Q5A/B(see FIG. 1) of the second stage. At low gains, the base voltage to gainrelationship approaches logarithmic, but near maximum gain, therelationship deviates significantly. The gain function is alsotemperature dependent. This control circuit of FIG. 6 includes areference amplifier to mimic the operation of the second stage of thepreamplifier. The reference amplifier is Q1A/B, Q2A/B and Q3A/B. Thegain voltage derived from this reference amplifier sets the second stageto the commanded gain.

To implement second stage gain control, the temperature dependentrelationship between base-emitter voltage and emitter (or collector)current is used. The second stage is controlled to route a fraction ofthe total emitter current into the output path on one of the collectors.The gain control voltage compensates for temperature dependence. Thecircuit for implementing the temperature compensation is shown in FIG. 7and outputs the control signal TEMP_COMP_TGC2. This control signaloperates over a voltage range of about 0V (e.g., the temperaturedependent point) to 1.7V. 1.7V corresponds to the maximum gain (i.e.,unity current gain). The low gain voltage corresponds to a second stagegain of about −34 dB and varies with temperature. The low gain voltageis about 0V at 25 C. Like the input stage gain control, the second stagegain control signal provides a gain slope of 20 dB/V, but other slopemay be used.

A portion of the control circuit for the second stage re-creates thecollector current that would be coming from the preamplifier input stageto the second stage. Q5A/B, Q6A/B, and Q7A/B are configured as apreamplifier input stage. U27A/B and Q8A/B mimic the vb voltagegenerator used on the preamplifier. The vb output generated may be alsoused in the control circuit for the input stage.

The transistors Q1A/B, Q2A/B, and Q3A/B emulate the function of the gainsteering pairs of transistors Q4A/B and Q5A/B of the second stage of thepreamplifier. The gain is set by controlling the relative emittervoltage of the pair of transistors Q1B/Q3A relative to the pair oftransistors Q2B/Q3B. Because of the relationship between base-emittervoltage and emitter current, current gain is controlled as theexponential of the differential base-emitter voltage. The pair oftransistors QIA/Q2A represents a shunt path carrying current necessaryto create a differential emitter voltage between the other two pairs oftransistors Q1B/Q3A and Q2B/Q3B. In alternative embodiments, three orother number of transistors implement this function. The threetransistors should be reasonably well matched. Since matched transistorsmay be available only in matched pairs, six transistors Q1A/B, Q2A/B,and Q3A/B may be used as shown. By intermingling the three pairs, themismatch between any two positions may be reduced.

The feedback loop around amplifier U9B creates the base voltage (vg2)relative to 2.25V necessary to create a voltage differential between theemitters of Q1A/Q1B/Q2A/Q3A and Q2B/Q3B. This voltage differentialresults from the amount of shunt current flowing in Q1A/Q2A. This shuntcurrent is the difference between the emitter currents of pairs Q1B/Q3Aand Q2B/Q3B. In the second stage amplifier, this difference current isthe amount of signal current that is shunted away from the signal path,and therefore represents attenuation. The difference current isproportional to the exponent of the difference voltage between theemitters of Q1A/Q1B/Q2A/Q3A and Q2B/Q3B. This accommodates thelogarithmic scaling used for the control signal TEMP_COMP_TGC2. When theshunt current in Q1A/Q2A is zero, there is no attenuation of the signal,and the gain of the second stage is unity. Also, the voltagedifferential between Q1A/Q1B/Q2A/Q3A and Q2B/Q3B is also zero. Whenthere is significant shunt current, the gain of the second stage issignificantly reduced. Likewise, there is a significant differentialvoltage difference between Q1A/Q1B/Q2A/Q3A and Q2B/Q3B. U8A/B are usedto buffer the emitter voltages of Q1A/Q1B/Q2A/Q3A and Q2B/Q3B to drivethe inputs of the differential amplifier U9B without disturbing thecurrent flows in the emitter circuits.

FIG. 7 shows the control circuit for splitting the TGC command signalinto two parts, one for the input stage gain control (TGC1) and theother for the second stage gain control (TGC2). The control circuit ofFIG. 7 operates to first increase one of the input and output stages toa maximum gain and then increase the other one of the output and inputstages to a maximum gain. Starting from minimum gain, the controlcircuit ramps the second stage gain (TGC2) from −34 dB to 0 dB, and thenramps the input stage gain (TGC1) from minimum to maximum, nominally a16 dB span. By applying the gain variation to the second stage of thepreamplifier first, the input impedance of the preamplifier is held at alowest value through most of the gain control range. Only at the deeperdepths does the gain and therefore input impedance of the input stage ofthe preamplifier rise. Such a control scheme is appropriate for atransducer whose temporal response is optimized with a low impedanceload. Control circuits with other apportionment of the gain between theinput and output stages may be used. For example, the gain of the inputgain stage increases to a maximum gain first, and then the gain of theoutput gain stage increases. In this way, the input impedance is higherover most of the gain control range and low only at the lowest gains(shallower depths) where the signal levels are the highest. Such ascheme would be appropriate for a transducer whose temporal response wasoptimized in driving a high impedance, since a low input impedance isused only when necessary for high signal levels. The TGC command issplit between two output signals by the clipping action of Q30A/B. For aTGC voltage between 0V and 1.7V, Q30A is off, so the voltage at thepositive input to U28A is the same as the TGC voltage. U28A buffers thisvoltage to create TGC2, the gain control command for the second stageamplifier. It is temperature compensated to offset the temperaturedependence of the second stage amplifier and its control referencecircuit shown in FIG. 6. RT1 an U24B and resistors R18, R19 and R21apply the appropriate temperature compensation. TGC1 is generated bydifferential amplifier U23B. TGC2 is driven into the non-inverting inputof differential amplifier U23B. Below a TGC voltage of 1.7V, TGC2matches TGC, so the same voltages are being fed into the inverting andnon-inverting inputs of U23B, and therefore its output voltage does notchange over this range. It sits at the positive DC voltage thatcorresponds to minimum input stage gain (approximately 0.54V for thisimplementation). For TGC voltages above 1.7V, the input to U28A clips to1.7V and so its output, the non-inverting input to U23B, stops rising.Above a TGC voltage of 1.7V, the output of U23B (TGC1) drops in voltageas TGC rises. As mentioned previously, the input stage gain and inputimpedance rise as the TGC1 command voltage drops in voltage. The scalingof TGC1 is the same as TGC, 20 dB/V

The TGC command signal varies between 0 and 2.5V, but may have othervalues or ranges. The total TGC range is 50 dB (e.g., 20 dB/V for0-2.5V), but greater or lesser ranges may be used.

A method is provided for varying an input impedance of a preamplifierused for ultrasound imaging or other uses. The method is implemented bythe preamplifier and/or the control circuits of FIGS. 1-7.Alternatively, the method is implemented by different preamplifierand/or control circuits.

Contribution to an input impedance of a preamplifier from a lower inputimpedance amplifier to a higher input impedance amplifier transitions asa function of time. The lower and higher input impedance amplifiersbeing connected in parallel in the preamplifier. The transition variesthe input impedance of the preamplifier. By gradually varyingcontribution to the preamplifier input impedance between the lower andhigher input impedance amplifiers, a smooth input impedance function isprovided over time. For example, different input impedances are providedfor ultrasound signals from two or more different depths. The smoothvariation or a small incremental change in input impedance associatedwith adjacent depths may avoid artifacts.

Both the lower and higher input impedance amplifiers contribute to theinput impedance of the preamplifier at a same time. A portion of therange of variation in input impedance may result from use of one or theother of the lower and higher input impedance amplifiers. A crossover ormid region of the range includes contribution to the input impedance byboth lower and higher input impedance amplifiers. By varying biases forthe lower and higher input impedance amplifiers, the input impedance iscontrolled. By operating the lower and higher input impedance amplifiersat a same time (i.e., both have biases in an active range), the inputimpedance of the preamplifier is a function of the relativecontributions of the lower and higher input impedance. Changes in thebias level alter the contribution.

The preamplifier amplifies ultrasound signals received from anultrasound transducer. During a receive event, signals from differentdepths are applied to the preamplifier at different times. The inputimpedance of the preamplifier transitions or varies during at least aportion of the receive event. Ultrasound signals associated withdifferent depths are applied to the preamplifier with different inputimpedance settings.

Acoustic signals attenuate during propagation. Received signalsrepresenting deeper depths may have smaller amplitude. To account forthe attenuation, a time varying gain is applied to the ultrasoundsignals. In one embodiment, the received signals are gain correctedafter pre amplification. In another embodiment, the received signals aregain corrected by pre amplification. The preamplifier amplifies thereceived signals as a function of time gain control. The gain of thepreamplifier varies as a function of the transition in input impedance.Alternatively, the gain is independent of the variation in inputimpedance. In another alternative embodiment, the gain of thepreamplifier varies as a function of the transition in input impedancefor a portion of the gain control range and is independent of thetransition in input impedance for a different portion of the gaincontrol range. The gain variance applied by the preamplifier may accountfor the entire range of time gain control or only a portion of therange.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

1. A variable input impedance preamplifier for ultrasound imaging, thepreamplifier comprising: a higher input impedance amplifier; a lowerinput impedance amplifier connected in parallel with the higher inputimpedance amplifier, the higher and lower input impedance amplifiersbeing parallel inputs to the variable input impedance preamplifier; andan ultrasound transducer connected with the higher and lower inputimpedance amplifiers.
 2. The preamplifier of claim 1 wherein the higherinput impedance amplifier comprises a common emitter amplifier.
 3. Thepreamplifier of claim 1 wherein the lower input impedance amplifiercomprises a common base amplifier.
 4. The preamplifier of claim 3wherein the higher input impedance amplifier comprises a common emitteramplifier, the lower input impedance amplifier having an input impedanceof about 25 ohms or less.
 5. The preamplifier of claim 1 wherein thelower and higher input impedance amplifiers are differential amplifiers;further comprising: a transformer having two outputs each connected withthe lower and higher input impedance amplifiers and having an inputconnected with the ultrasound transducer.
 6. A variable input impedancepreamplifier for ultrasound imaging, the preamplifier comprising: ahigher input impedance amplifier; a lower input impedance amplifierconnected in parallel with the higher input impedance amplifier; and anultrasound transducer connected with the higher and lower inputimpedance amplifiers; a control circuit operable to vary first andsecond bias currents to the lower and higher input impedance amplifiers,respectively.
 7. The preamplifier of claim 6 wherein the control circuitis operable to gradually transition between the lower and higher inputimpedance amplifiers as a function of the first and second biascurrents, both the lower and higher input impedance amplifiers operableto be active during at least a same portion of the transition.
 8. Thepreamplifier of claim 6 wherein a gain of the preamplifier varies as afunction of the first and second bias currents, the control circuitoperable to vary the gain as a function of time relative to reception ofacoustic echoes by the ultrasound transducer.
 9. The preamplifier ofclaim 6 wherein the control circuit is operable to drive a base of thelower input impedance amplifier with an attenuated and voltage levelshifted signal as compared to a time gain control signal.
 10. Thepreamplifier of claim 6 wherein the control circuit comprises atemperature compensation circuit.
 11. The preamplifier of claim 1further comprising a current steering amplifier connected with the lowerand higher input impedance amplifiers.
 12. The preamplifier of claim 11further comprising a control circuit operable to control a first gain ofa first stage comprising the lower and higher input impedanceamplifiers, the first gain being as a function of input impedance, andoperable to control a second gain of the current steering amplifier. 13.The preamplifier of claim 1 further comprising an ultrasound receivebeamformer channel connected with the lower and higher input impedanceamplifiers.
 14. A variable input impedance preamplifier, thepreamplifier comprising: a higher input impedance amplifier; a lowerinput impedance amplifier connected in parallel with the higher inputimpedance amplifier, the higher and lower input impedance amplifiersbeing parallel inputs to the variable input impedance preamplifier; anda control circuit connected with the higher and lower impedanceamplifiers, the control circuit operable to control an input impedanceof the preamplifier in conjunction with a variance in gain of thepreamplifier.
 15. The preamplifier of claim 14 wherein the inputimpedance and gain are controlled as a function of bias currents of thehigher and lower input impedance amplifiers.
 16. The preamplifier ofclaim 14 wherein the higher input impedance amplifier comprises a commonemitter amplifier, the lower input impedance amplifier comprises acommon base amplifier.
 17. The preamplifier of claim 14 wherein thecontrol circuit is operable to gradually transition between the lowerand higher input impedance amplifiers as a function of first and secondbias currents, both the lower and higher input impedance amplifiersoperable to be active during at least a same portion of the transition.18. A method for varying an input impedance of a preamplifier used forultrasound imaging, the method comprising: transitioning contribution tothe input impedance of the preamplifier from a lower input impedanceamplifier to a higher input impedance amplifier as a function of time,the lower and higher input impedance amplifiers being connected inparallel; and amplifying ultrasound signals with the preamplifier whiletransitioning.
 19. The method of claim 18 further comprising: varying again of the preamplifier as a function of the transitioning.
 20. Themethod of claim 18 wherein transitioning comprises gradually varyingbetween the lower and higher input impedance amplifiers, both the lowerand higher input impedance amplifiers contributing to the inputimpedance at a same time over a range of the input impedance.
 21. Themethod of claim 18 wherein transitioning comprises varying first andsecond biases for the lower and higher input impedance amplifiers,respectively.
 22. The method of claim 18 wherein amplifying comprisesamplifying as a function of time gain control.
 23. The preamplifier ofclaim 1 wherein the lower input impedance amplifier is connected inparallel with the higher input impedance amplifier between theultrasound transducer and an ultrasound receive beamformer channel.